The invention relates to glass that is transmissive at wavelengths below 193 nm, in particular, a photomask silicon oxyfluoride glass suitable for use in the Vacuum Ultraviolet (VUV) 157 nm wavelength region.
The present invention relates to a device and method for measuring state of polarization (SOP), and more particularly to a device and method for rapidly measuring SOP at different wavelengths.
Polarization mode dispersion (PMD) in single-mode optical fibers is a recognized source of bit errors in high-speed optical communication systems. PMD may cause optical pulse broadening, or pulse deformation in general. As such, PMD may limit the bit rate that can be achieved with a given optical communication system that does not compensate for PMD. As pulses broaden, the individual bits eventually are no longer distinguishable and the communication system ceases to properly function.
In the theoretical field, Poole and Wagner introduced a model for polarization dispersion in single-mode fibers based on so-called principle states of polarization (PSPs) (see Poole and Wagner, xe2x80x9cPhenomenological Approach to Polarization Dispersion in Long Single-mode Fibres, Electronics Letters, volume 22, pp. 1029-1030, 1986xe2x80x9d). In this approach, the broadening induced by first order PMD is caused by the propagation time difference, known as differential group delay (DGD), between the input pulse projections along the two PSPs. Second order PMD also results in pulse defamation, and is due to the frequency dependence of the PSPs and the DGD (see C. D. Poole and C. R Giles, xe2x80x9cPolarisation-dependent Pulse Compression and Broadening Due to Polarisation Mode Dispersion in Dispersion-shifted Fibres, Optics Letters, volume 13, pp. 155-157, 1988xe2x80x9d). Second order PMD induced pulse broadening may be a significant obstacle to using high bit-rates of, for example, 40 Gbit/s and higher.
One of the key components of an effective PMD compensator is the detection mechanism for determining the impairment, the compensator is to mitigate. To date, an effective device and method for detecting second order DGD for an optical communication system is not generally available.
One aspect of the present invention is a system for measuring the state of polarization of an optical signal. The system includes a local oscillator providing a local optical signal. The local oscillator is tunable such that the frequency of the local optical signal can be selectively varied. A polarization transformer is operatively connected to the local oscillator to transform the local optical signal to a selected one of a plurality of polarizations. A coupler is operatively connected to the polarization transformer. The coupler is adapted to combine an optical signal from the polarization transformer with an input optical signal from an optical system. A heterodyne receiver is operatively connected to the coupler. The heterodyne receiver is configured to detect the beat frequency of the signal from the coupler and provide an output signal that can be utilized to determine the state of polarization of an input optical signal.
Another aspect of the present invention is a method of measuring the state of polarization of an optical signal in an optical line. The method includes providing a local optical signal at a first selected frequency. The local optical signal is transformed to a first selected polarization. The local optical signal having the first selected polarization is combined with an input optical signal from the optical line to form a first combined signal having a first amplitude at the beat frequency. The amplitude at the beat frequency of the combined signal is then detected, and the local optical signal is transformed to a second selected polarization. The local optical signal having the second selected polarization is combined with an input optical signal from the optical line to form a second combined signal having a second amplitude at the beat frequency. The amplitude at the beat frequency of the second combined signal is then detected, and the local optical signal is transformed to a third selected polarization. The local optical signal having the third selected polarization is combined with an input optical signal from the optical line to form a third combined signal having a third amplitude at the beat frequency. The third amplitude at the beat frequency of the third combined signal is detected. The state of polarization for the first selected frequency of the local optical signal between the input pulse projections is calculated based, at least in part, upon the third amplitude at the beat frequency. The frequency of the local optical signal is changed to a selected second frequency that is different than the first selected frequency. The local optical signal is transformed to first, second, and third polarizations, and the state of polarization is calculated for the second selected frequency.
Yet another aspect of the present invention is a method of measuring the state of polarization of an optical signal and an optical line. The method includes providing a local optical signal having a frequency that can be set at a selected one of a plurality of frequencies. The local optical signal is combined with an input optical signal from the optical line to produce a combined signal having a beat frequency. The frequency and polarization of the local optical signal are varied. The differential group delay of the optical signal in the optical line at a plurality of frequencies and polarizations of the local optical signal is calculated based, at least in part, upon the resulting amplitude at the beat frequencies of the combined signal.
Yet another aspect of the present invention is a device for measuring the state of polarization of a system optical signal. The device includes a local light source generating a general local signal. The local light source is configured to selectively vary the frequency and polarization of the local optical signal. The device includes a coupler that is adapted to combine the local optical signal with a system optical signal to produce a combined signal having a beat frequency. The device further includes a detector configured to provide a signal corresponding to the beat frequency to permit determination of the differential group delay across the bandwidth of the system optical signal.
Yet another aspect of the present invention is an optical communication system including an optical transmitter, an optical receiver, and a fiber optic line extending between an operatively interconnecting optical receiver to the optical transmitter to permit transmission of a system optical signal therebetween. The optical communication system also includes a state of polarization detecting device having a local light source providing a local optical signal. The local light source is configured to selectively vary the frequency and polarization of the local optical signal. The polarization mode dispersion detecting device also includes a coupler operatively interconnecting the local light source to the fiber optic line to produce a combined signal having a beat frequency. A detector is configured to provide a signal corresponding to the beat frequency to permit determination of the state of polarization across the bandwidth of the system optical signal.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.